This invention is an automatic tone density controller for maintaining the correct toner density in a xerographic imaging system and specifically is a miniature development station which coats a piece of NESA glass with liquid toner, and tests the resultant optical density.
In a typical xerographic system, dry toner and carrier particles are applied to an exposed plate. Because of differing electrical charges between the toner and plate, toner is stripped from the carrier particles and deposited on the plate, and, periodically, toner must be added to the carrier. Several automatic toner density controllers have been invented to do this.
The most common controller uses a charged plate of NESA glass to attract toner, and optically measures the density of toner attracted to the plate. This method suffers from inaccuracies due to the build-up of toner and other contaminants on the glass and other parts of the sensor assembly.
In copiers using liquid toner, it is common for the image density to be controlled indirectly by monitoring the turbidity or optical density of the liquid developer. This is accomplished by sending the developer through a glass tube and by measuring the transmission density electro-optically. If the transmission density has fallen below a reference density, toner concentrate will be added automatically until the sensed reading equals the reference value.
This relatively simple liquid toner controller has two basic problems. First, the glass tube tends to collect toner on its inside walls. Since the sensor is looking for a constant level of light transmitted through the walls and developer fluid flowing between them, any toner build-up on the walls results in an unwanted decrease in toner density. Second, the amount of toner deposited on a photoreceptor, and thus the image density, depends mainly on the particle concentration, charge-to-mass ratio (Q/M), mobility of the toner particles in the carrier fluid and the conductance of the carrier fluid. For example, for the same toner concentration, lower Q/M toner produces lighter images than higher Q/M toner, and toners of higher mobility or conductance generate darker images than toners that have a measurably lower mobility or conductance. To overcome a drop in Q/M, toner concentrate could be added to maintain an average image density if the change in Q/M could be detected. Other factors that affect density are the fountain flow rate, fountain gap, fountain field voltage and plate speed and time.
Since most liquid developers, especially highly sensitive ones as are being used in the preferred embodiment, exhibit temporal changes of Q/M, mobility and conductance, a toner density controller compensating for all of the above-mentioned property changes needed to be invented. The need is severe in a mammography system since mammography radiologists look predominately for changes over long periods of time in breast morphology, thus highlighting the need for consistent image density as patients return for repeat examinations.
In the context of an automatic system for the development of mammography x-ray images exposed on xerographic plates, there is a more severe constraint. Because of the hazard of x-rays, the patient must be exposed to a minimum amount of radiation. Therefore, there must be a high level of confidence in the system before the plate is exposed the first time, so that there will be no repeat exposures.